1. Technical Field
The present invention relates generally to satellite communication systems. More particularly, the present invention is directed to a system and technique for digital beamforming based on predetermined grid information.
2. Discussion
A primary interest of telecommunications technology is enhancing the features offered by communication systems and expanding the coverage area of these systems. The principal goal behind developing next generation communication systems is either to improve the transmission fidelity, to increase the data rate, or to increase the distance between relay stations. In an effort to accomplish these objectives, earth-based (or terrestrial) communication systems have evolved over the years to incorporate high-speed electro-optic equipment and optical fibers. The result has been a substantial increase in the fidelity and speed of transmissions. Optical communication has therefore allowed quality and grade of service to reach new heights. Optical communication systems have not provided, however, a solution to the increasing mobility of today""s society and the need for global communication.
First generation satellite telecommunication systems are now capable of providing communications services around the world using RF signals. These sophisticated communications satellites transmit many telephone, data and television signals simultaneously over long distances. Satellite communication systems have therefore proven to be well suited for the increasing demand for wireless communications and global transmissions. Substantial competition still exists, however, between satellite and terrestrial communication systems in the arena of quality of service. It is therefore desirable to improve the transmission fidelity and data rate of satellite-based communication systems and thereby improve the quality and grade of service.
Conventional satellite communications networks employ a plurality of interconnected space vehicles, or satellites, in an effort to provide satellite communication channels to subscriber equipment via transmitted beams. Attenuation in the beams is common and is generally due to environmental effects such as trees and rain. In a given service area, it is therefore necessary to specify acceptable levels of attenuation in the beam transmitted from the satellite and received by the subscriber equipment (e.g., pagers, mobile phones, etc.). Attenuations are specified in terms of (ink margins and can be defined for different types of communication links such as in-building, in-vehicle, and mobile outdoor links.
In the past, satellites used fixed antenna beams to provide communication channel coverage over the entire satellite footprint. This technique of providing communications links is potentially inefficient due to the need to provide coverage over the entire satellite footprint while providing high gain antenna beams to improve link quality and subscriber equipment characteristics. For example, the additional necessary hardware required to form enough fixed high gain antenna beams to cover the satellite footprint will substantially increase the satellite size, weight, power, and processing requirements. The communications electronics payload is also affected because of the added complexity of the switching, routing, and processing of the communication channels associated with these beams.
Recent trends have been toward providing an increasing number of narrow antenna beams to improve spectral efficiency, link quality, and improve subscriber equipment characteristics (e.g. size, weight, power, channel throughput, etc.). Thus, satellites are configured with an array of antenna beams that can provide hundreds of signals to predetermined areas of the satellite footprint. This technique provides a means to deal with increased antenna complexity while providing complete coverage within the satellite footprint.
Implementation of narrow beamforming, however, has been inflexible and has failed to make sufficient use of information regarding the service area. For example, link margins and service areas change drastically from region to region while conventional beam configurations remain fixed as the satellite orbits the earth. This is an issue of considerable concern because processing power is wasted in areas of sparse population such as uninhabited rain forests. In order for narrow beamforming to be a viable alternative, the satellite must be able to adjust the size, location, and content of each beam in the footprint as the communication requirements change. Providing such a real-time response to consumer demand is crucial in closing the gap between satellite and terrestrial quality and grade of service.
It is therefore desirable to provide maximum coverage at the necessary link margins. It is further desirable to provide variable link margin services in areas capable of supporting these margins and or willing to pay premium prices for higher link margin services. Another concern with the use of multiple beams is due to the effect of the number of beams on the size of the antenna array. Conventional approaches have effectively xe2x80x9cblanketedxe2x80x9d the satellite footprint with beams of fixed strength and thereby required the use of large, bulky antenna arrays. The use of digital beamforming technology would allow selective formation of beam sizes and significantly reduce power and processing consumption by reducing the number of required beams.